The invention relates to an electric lamp comprising:
a light-transmissive lamp vessel sealed in a vacuumtight manner, with a quartz glass wall enclosing a space accommodating an electric element which is connected via an inner current conductor projecting from the wall of the lamp vessel into the space to a metal foil completely embedded in the wall;
at least an outer current conductor having a first end, a second end and a diameter, said first end being connected to the metal foil and being at least substantially completely enclosed by the wall of the lamp vessel, and said second end projecting from the wall to the exterior;
an anchored part comprising a deformed portion of the first end of the outer current conductor in co-operation with the enclosing wall.
A lamp of this type is known from DE-C-1 016 848 to which GB 721,621 corresponds. In the manufacture of the lamp, a seal is made and one or more of the outer current conductors is partly incorporated in the wall. At the area of the seal, the quartz glass is caused to soften in the presence of the metal foil and the outer current conductor which have a connection with each other, for example a weld. The quartz glass reaches a temperature of more than 1900xc2x0 C. Subsequently, the seal thus formed is cooled. The outer current conductor shrinks due to its relatively high linear coefficient of thermal expansion (approx. 50*10xe2x88x927 Kxe2x88x921) more strongly than the quartz glass, glass with an SiO2 content of at least 95% by weight (linear coefficient of thermal expansion approximately 6*10xe2x88x927 Kxe2x88x921) in which it is embedded. A capillary space around this outer current conductor is thus obtained, as a result of which the outer current conductor is incorporated in the wall of the lamp vessel with some freedom of movement. Due to the capillary space around the outer current conductor, the connection between the metal foil and the outer current conductor is the only connection of the outer current conductor with the lamp. In the known lamp, the outer current conductor in the wall of the lamp vessel has an anchored part so that the freedom of movement of the outer current conductor in the wall of the lamp vessel is limited. Consequently, the weak connection between the metal foil and the outer current conductor of the lamp remains at least substantially unloaded in the case of a mechanical load of the outer current conductor. A drawback of the known lamp is that the anchored part of the outer current conductor in the wall of the lamp vessel is obtained by cutting a groove in the first end of the outer current conductor. A drawback of cutting is that it is a cumbersome operation and involves contamination. Further drawbacks of the known lamp are a reduced conductivity and a decrease of the mechanical strength of the outer current conductor due to the groove provided therein.
It is an object of the invention to provide an electric lamp which has a simple construction and can easily be made, and which obviates the above-mentioned drawbacks.
According to the invention, the deformed portion of the anchored part is a plastically deformed portion of the first end. Plastic deformation of the outer current conductor is an operation which is usually free from loss of material because, generally, material is displaced and, in essence, nothing is removed so that the operation is free from contamination. Plastic deformation may be a relatively simple operation which is cost-effective. The lamp may thus be manufactured in a relatively simple, inexpensive and clean way. The plastically deformed portion has the further advantage that the outer current conductor at the area of the deformed portion has a surface which also defines the maximum conductivity through the outer current conductor, which, in contrast to the known lamp, is at least substantially not reduced.
In a further embodiment, the outer current conductor at the area of the deformed portion has at least two diameters which are mutually different in size. Such an embodiment of the anchored part has the advantage that the outer current conductor is limited in its freedom of movement also as regards a torsion load of the outer current conductor.
In a favorable embodiment, the anchored part comprises a flattened portion of the first end. The outer current conductor with a flattened portion can easily be formed with a satisfactorily reproducible quality in one rapid stroke by means of (possibly profiled) dies. Since the occurrence of, for example, a sharp kink in the first end is thus prevented, the risk of breakage is relatively small, which is an advantage.
In another embodiment, the anchored part comprises a bent portion of the first end. The anchored part with a bent portion is an alternative to the anchored part with a flattened portion and can also be made easily and rapidly. Moreover, the anchored part with the bent portion has the advantage that the risk of longitudinally splitting the material of the outer current conductor, as may occur, for example, during flattening, is further reduced.
The preferred embodiment of the lamp, i.e. an anchored part with a flattened or a bent portion, is dependent on the sensitivity of the material of the outer current conductor to breakage or splitting during plastic deformation of the outer current conductor.